Manhattan Project calculates the effects of The Gadget

24th April 1943: Manhattan Project calculates the effects of The Gadget

The effect of the radioactive fission products depends entirely on the distance to which they are carried by the wind. If 1 kg of fission products is distributed uniformly over an area of about 100 square miles, the radioactivity during the first day will represent a lethal dose (=500 R units): after a few days, only about 10 R units per day are emitted, If the material is more widely distributed by the wind, the effects of the radioactivity will be relatively minor.

In March 1943, DuPont began construction of a plutonium plant on a 112-acre (0.5 km2) site at Oak Ridge. Workers load uranium slugs into the X-10 Graphite Reactor.
In March 1943, DuPont began construction of a plutonium plant on a 112-acre (0.5 km2) site at Oak Ridge. Workers load uranium slugs into the X-10 Graphite Reactor.

The 24th April 1943 saw an important milestone in the the “Manhattan Project”. “Manhattan District” had originally just been the U.S Army code name for their involvement in aspects of the project. This was the name that was gradually adopted to refer to the whole official programme for the “Development of Substitute Materials”, the atomic weapons programme, designated “Tube Alloys” by the British.

From very modest beginnings that had evolved from the the first theoretical understanding of the power of atomic energy the programme, which had only begun in 1941, had developed into a major industry. During the course of the war it would cost $2 billion – around $26 billion in 2013 values – and employed over 130,000 people in a wide variety of sites around the United States. Very few of these people knew what they were actually working on.

Only a small group of individuals at the top had a complete overview of the work they were doing. Between 15th-24th April they met in conference to discuss the outcomes of the different strands of research. They were now in a position to understand how much nuclear material they could produce and what the destructive power of a potential weapon was likely to be. The testing of a real atomic weapon device or “Gadget” still remained a long way off:

OUTLINE OF PRESENT KNOWLEDGE

Robert Oppenheimer

Energy Release: The destructive effect of the gadget is due to radiative effects and the shock wave generated by the explosion. . The shock wave effect seems to extend over the biggest area and would be, therefore, most important. The area devastated by the shock wave is proportional to the 2/3 power of the energy release and may be simply calculated by comparing the energy release with that of TNT. If the reaction would go to completion, then 50 kg of [isotope] 25 would be equivalent to 10 tons of TNT. Actually it is very difficult to obtain a large percentage of the potential energy release.

Detonation: The second major difficulty facing us is connected with the question of detonation. . It is important that no neutron should start a premature chain reaction. . . Possible sources of neutrons are 1) Cosmic ray neutrons . . . and 2) Spontaneous fission neutrons. . .

EXPECTED DAMAGE OF THE GADGET

Hans Bethe

Comparison with TNT: The most striking difference between the gadget and a TNT charge is in the temperatures generated. The latter yields temperatures of a few thousand degrees whereas the former pushes the temperature as high as [tens of millions of degrees]. . . .

The actual damage depends much on the objective. Houses begin to be smashed under shocks of 1/10 to 1/5 of an atmosphere. For objects such as steel supported buildings and machinery, greater pressures are required and the duration of the shock is very important. If the duration of the pressure pulse is smaller than the natural vibration period of the structure, the integral of the pressure over the duration T of the impulse is significant for the damage. If the pulse lasts for several vibration periods. the peak pressure is the important quantity. . . .

Other Damage: The neutrons emitted from the gadget will diffuse through the air over a distance of 1 to 2 km, nearly independent of the energy release. Over this region, their intensity will be sufficient to kill a person,

The effect of the radioactive fission products depends entirely on the distance to which they are carried by the wind. If 1 kg of fission products is distributed uniformly over an area of about 100 square miles, the radioactivity during the first day will represent a lethal dose (=500 R units): after a few days, only about 10 R units per day are emitted, If the material is more widely distributed by the wind, the effects of the radioactivity will be relatively minor.

See The Atomic Archive for original documents.

The Chindits march into the Burmese jungle

7th March 1943: The Chindits march into the Burmese jungle

It was also assumed that supplies would be dropped regularly, which turned out, after the first two drops, to be a false hope, not because of any shortage of aircraft, just that the enemy’s presence often made it impossible to pick and choose time and place. As a result, for most of the expedition, one day`s rations had to last at the very least for three, and too often much longer.

Chindits with their mules carrying supplies make their way through the jungle.
Chindits with their mules carrying supplies make their way through the jungle.

In Burma, now occupied by the Japanese, the British were experimenting with unconventional methods of warfare. Colonel Orde Wingate had won support for the development of a deep penetration guerrilla force that would march far into the jungle, way behind the front line. There they would disrupt the enemy’s lines of communication by blowing up railway lines, as well as attacking Japanese troops. The new force soon became christened ‘Chindits’, after mythical Burmese creatures.

The conditions the troops were expected to live in were arduous enough. However, the problem of resupplying such troops by air was also experimental, and was to lead to further privations. The first column set off into the jungle in late February 1943 and received their first parachute drop of supplies in early March. Harold James was a nineteen year old officer on his first military operation:

The dropping area was strewn with ration tins, parachutes and mule fodder, and the men soon got to work collecting the stores. Burmese from the nearest village were called in to help, and in return were given the parachutes which they greatly prized, cloth being a scarcity. We soon learned that valuable information could often be obtained for a piece of parachute. The Gurkhas made handerchiefs and ration bags for themselves, and lanyards from the cords.

Four tins were dropped with each parachute, padded with a shock absorber fastened by thick webbing – although this did not always work if the parachute should break loose. The four tins could conveniently be loaded each side of a mule, allowing extra rations to be carried as reserve, and, as on this occasion, the supplies could be easily transported from the dropping zone to our camp for distribution to the men.

The hard scale daily ration laid down was:

Shakapura biscuits l2oz
Cheese 2oz
Milk powder 1oz
Raisins and almonds 9oz
Tea 3/4 oz
Sugar 4oz
Acid drops or chocolate 1oz
Salt 1/2oz
Cigarettes 2 packets of 10
Matches 1 box

To imagine that men could keep fit on a ration of this nature for three months of marching through very rough country, fighting, physically and mentally extended, is beyond belief, and would seem to show a definite lack of imagination in planning the ration menu.

But the expedition was heading into unexplored areas of logistics, and presented problems which had to be solved by guesswork before hard experience could produce the correct results.

There was no meat, although tins of corned beef were dropped later, and on occasion corned mutton for the Gurkhas. But it was bulky, and went bad quite quickly, so had to be eaten more or less in one sitting. The parachute ration was supposed to be supplemented from local produce, which often proved impracticable.

With over 300 mouths to feed, very few villages could provide more than a few mouthfuls of rice per person, and the odd chicken or egg. Some columns were lucky in coming across an extra friendly village which would be more helpful – but seldom more than once during the expedition.

The idea behind the rations selected was that they contained nothing that required cooking, except water for tea, since it was expected that troops would not be able to count on more than twenty minutes for meals. In practice
we rarely had to rush our meals.

It was also assumed that supplies would be dropped regularly, which turned out, after the first two drops, to be
a false hope, not because of any shortage of aircraft, just that the enemy’s presence often made it impossible to pick and choose time and place. As a result, for most of the expedition, one day`s rations had to last at the very least for three, and too often much longer.

A great deal of will power was needed to limit the daily intake.

See Harold James: Across the Threshold of Battle